1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly, the present invention relates to an electrophotographic photosensitive member having a specified surface layer. The present invention also relates to an electrophotographic apparatus and a device unit employing the above electrophotographic photosensitive member.
2. Related Background Art
The electrophotographic photosensitive member is not only required to have sensitivity, and electric and optical properties suitable for the electrophotographic process in which the member is employed, but for the photosensitive member to be used repeatedly it is required to have sufficient resistance to the external electric and mechanical forces applied to the member in the cycle of corona charging, toner development, image transfer onto a recording medium, and surface cleaning. Specifically, the photosensitive member is required to be resistant to abrasion and scratching of the surface caused by sliding friction at the surface, and to the deterioration due to the action of ozone or nitrogen oxides. Additionally, the surface of the electrophotographic photosensitive member is required to have excellent cleaning performance in order to prevent undesired toner adhesion.
To satisfy such requirements, it is known to provide a surface protection layer which is mainly composed of a resin on the photosensitive layer. For example, Japanese Patent Application Laid-Open No. 62-272282 discloses a protection layer containing a fluorine-containing resin particles and a charge-transporting substance to improve the mechanical durability and potential stability.
Generally, a film made from a resin of a lower molecular weight shows satisfactory adhesiveness to the other layer, but is less satisfactory in hardness and durability. On the contrary, a film made from a resin of a high molecular weight tends to have larger internal stress, resulting in crack formation in the film or brittleness of the film, particularly when a rather thick film like a photosensitive layer is formed.
With the increasing demands for image quality and durability, electrophotographic photosensitive members having more enhanced electrophotographic characteristics and much higher durability have been studied.